I490 THALLOPHYTES. 



give rise, both in the sea and in fresh waters, to very extensive 

 deposits, the general nature of which is that of a fine siliceous mud, 

 sometimes arranged in thin laminae. Of this nature are the deposits 

 known as " Kieselguhr," " Bergmehl," " Tripoli," and " Polir- 

 schiefer," or, simply, as " Diatomaceous Earths." Such deposits 

 may cover wide areas, and may attain a very considerable thickness ; 

 and, under an erroneous idea as to their true nature, they have been 

 sometimes spoken of as " Infusorial Earths." 



As regards their distribution in time, the cases of Diatoms have 

 been stated to occur in the ashes of the coal of the Coal-measures, 

 but this observation has not been supported by subsequent investi- 

 gations. In the Middle and Upper Trias whole strata are some- 

 times largely made up of elongated, parallel-sided, siliceous tubes, 

 with flattened sides and rounded ends, which have been described 

 under the generic name of Bactryntum, and which may possibly be 

 Diatoms. If this be their real nature, they are comparatively gigan- 

 tic members of the class, as they attain a length of from two to four 

 millimetres. Leaving these problematical fossils out of sight, un- 

 doubted Diatoms occur in the Upper Chalk, though only a few 

 forms are known. The regular " Diatomaceous Earths " are all of 

 Tertiary or Post-Tertiary age ; and among the best known deposits 

 of this epoch may be mentioned the numerous beds of " Kieselguhr" 

 which occupy the sites of ancient lakes in various regions in the 

 northern hemisphere, the " Richmond Earth " of Virginia, the 

 " Tripoli " of Oran in Africa, and Bilin in Bohemia, and the lami- 

 nated " Polir-schiefer " of Cassel. 



Class II. Phycochromophyc^e. — The Algce included in this 

 class are unicellular or multicellular, usually bluish-green in colour, 

 and inhabit water or live in moist places on land. Reproduction 

 takes place by cell-division, or by the formation of asexually-produced 

 cells (" gonidia"). The integument is not hardened by the deposi- 

 tion of silica within it. In this class are included numerous widely 

 distributed forms of the Algce, such as the Oscillarice, the Nostocs, 

 and the Desmids. Owing to the soft nature of their outer covering, 

 no undoubted remains of this class of Algce. have been detected in 

 the fossil condition ; but remains of Nostoc are said to occur in strata 

 of Tertiary age, and the curious spherical and spined bodies which 

 are known as Xanthidia, and which are sometimes recognisable 

 under the microscope in thin sections of flint or chert, have been 

 supposed to be the spores of Desmids. 



Class III. Fucace^e. — This class includes the common brown 

 Sea-weeds, and comprises forms which are often of considerable size 

 and of various shape. The thallus is often foliaceous and much 

 branched, or may be more or less filamentous. The reproductive 

 organs are developed in special cavities or " conceptacles." The 



